The preparation of one enantiomer of optically active substituted 1,4-diols, though known in the literature, is carried out with tedious, time consuming methods. For example, S. Masamune et al., Journal of Organic Chemistry, 54, 1755 (1989), teaches the use of Baker's yeast for the reduction of 2,5-hexane dione to (S,S)-2,5-hexanediol in 50% yield based on a method originally disclosed by J. K. Lieser, Synthetic Communications, 13, 765 (1983). Lieser had reported a yield of 57%. Enzymatic reductions can generally be used to provide only one enantiomer of the desired product and can have limitations such as high substrate specificity, low product yields, long reaction times (144 hrs in the Lieser reference) or complex isolation procedures due to the usually highly dilute reaction mixtures (ca. 5 grams per liter in the Lieser reference).
The electrochemical coupling of carboxylic acids, i.e., 2 RCOOH.fwdarw.R-R+2 CO.sub.2 +H.sub.2 is known as Kolbe coupling.
U.S. Pat. No. 3,787,299 issued Jan. 22, 1974 discloses the Kolbe coupling of carboxylic acids and substituted carboxylic acids. The disclosed substituents, which may be in the .beta. position, include ester, acylamino, acyloxy, nitrilo, halo, aryl, alkyl, aralkyl or heterocyclic. There is no disclosure nor suggestion of the applicability to carboxylic acids with unprotected hydroxyl groups. There is no disclosure nor suggestion of the utility of this process for preparing optically active compounds with a high degree of enantiomeric purity.
G. E. Svadkovskaya et al., Russian Chemical Reviews, English Translation, 29, 161, 180 (1960), especially p 166, states that aliphatic hydroxy acids are not very suitable for the Kolbe reaction as the hydroxyl group is readily oxidized. "Negative results were obtained on electrolysing .beta.-hydroxy acids." "Formic acid, crotonaldehyde, and other oxidation products are obtained from beta-hydroxy butyric acid."
The Kolbe coupling of hydroxy substituted carboxylic acids is reported to be a low yield reaction by J. Haufe et al., Chem. Ing. Tech., 42, 170-5 (1970).
L. Rand et al., J. Org. Chem., 33, 2704 (1968) report the electrochemical coupling of 1-hydroxycyclohexylacetic acid in a maximum yield (9 experiments) of 40%. There is no suggestion of a route to higher yield processes. There is no suggestion of applicability of the reaction to optically active compounds nor of the fate of optical activity if it were applicable to optically active compounds.
Thus, D. Seebach et al., Helv. Chim. Acta, 68, 2342 (1985) protected the hydroxyl group of optically active beta hydroxy carboxylic acids by esterification or etherification prior to Kolbe coupling. These workers reported that racemization of the "protected" .beta.-hydroxy carboxylic acids did not occur during Kolbe coupling. There is no suggestion nor prediction of the fate of optical activity in the Kolbe coupling of "unprotected" beta hydroxy carboxylic acids.
By the process of the present invention is provided a high yield route to optically active 1,4-diols with a high degree of enantiomeric purity via the Kolbe coupling of optically active, "unprotected" beta hydroxy carboxylic acids with a high degree of enantiomeric purity in which racemization of the asymmetric carbon does not occur.